<ul class="list">
  <li class="list__item">
    <a class="my-button js-button" href="#">a要素で作ったボタン</a>
  </li>
  <li class="list__item">
    <button class="my-button js-button" type="button">button要素で作ったボタン</button>
  </li>
  <li class="list__item">
    <div class="my-button js-button">div要素で作ったボタン</div>
  </li>
</ul>

html {
  box-sizing: border-box;
}

*,
*::before,
*::after {
  box-sizing: inherit;
}

body {
  background-color: #f7f7f7;
  color: #333;
  font-family:
    'Helvetica Neue',
    'Segoe UI',
    'Hiragino Kaku Gothic ProN',
    'Hiragino Sans',
    Meiryo,
    sans-serif;
  padding: 5%;
}

.list__item {
  max-width: 320px; 
  
  & + & {
    margin-top: 1.5em;
  }
}

.my-button {
  appearance: none;
  background-color: transparent;
  border: 1px solid #878787;
  color: #555;
  cursor: pointer;
  display: block;
  font: inherit;
  max-width: 320px;
  padding: 1.2em 1.5em;
  position: relative;
  text-align: center;
  text-decoration: none;
  transition: background-color .3s, color .3s;
  width: 100%;

  &::after {
    border: 4px solid transparent;
    border-bottom: 4px solid;
    border-radius: 2px;
    border-right: 4px solid;
    bottom: 4px;
    content: '';
    display: inline-block;
    position: absolute;
    right: 4px;
  }

  &.focus-visible {
    background-color: #555;
    color: #f7f7f7;
  }
  
  @media (hover) {
    &:hover {
      background-color: #555;
      color: #f7f7f7;
    }
  }
}

.message-box {
  background-color: #cd5c5c;
  border-radius: 4px;
  box-shadow: 0 10px 12px -8px rgba(0, 0, 0, .35);
  color: #f7f7f7;
  font-size: .8rem;
  margin: 12px auto 0;
  min-width: 196px;
  padding: 1.4em 1.5em;
  position: relative;
  text-align: center;
  width: 60%;

  &::before {
    border: 8px solid transparent;
    border-bottom: 10px solid #cd5c5c;
    bottom: 100%;
    content: '';
    left: calc(50% - 8px);
    position: absolute;
  }
}

.js-focus-visible :focus:not(.focus-visible) {
  outline: none;
}

!function(){
  const button = document.querySelectorAll('.js-button');
  const messageBox = document.createElement('p');
  
  messageBox.className = 'message-box';
  messageBox.innerHTML = 'プッシュされました！';

  button.forEach((el) => {
    el.addEventListener('click', (e) => {
      e.preventDefault();
      e.currentTarget.parentNode.insertBefore(messageBox, e.currentTarget.nextElementSibling);
    });
  });
}();